In underground operations where the rock in the roof and walls is too weak to stand alone, it must be supported. Rock bolting has found increasing application in drift and other types of openings to hold the rock structure together, to reinforce strata, and to stabilize the structure by holding key pieces of rock in place. The use of rock bolts enables the tunnels to be kept clear of obstructions so as to more easily permit the use of heavy mechanized equipment.
A rock bolt is a threaded steel rod provided with means for anchoring in a pre-drilled bore hole in the rock. The anchoring of the bolts is effected by an expansion shell which has been inserted into the bore holes and through the wedge of which the bolts are threaded. As the bolt is threaded, the expansion shell is caused to expand so as to firmly grip the wall of the bore holes. A plate or other retainer can then be pulled against the rock with a nut on the threaded bolt.
A number of patents have been addressed to expansion shells and methods of making the same.
Canadian Pat. No. 211,834 discloses the method of forming an expansion shell from sheet metal by a simple punching or stamping process. All but the upper surface of the blank is corrugated on diagonal lines. The blank is then rolled around a central axis into a tapered tube. The thickness of the walls is increased by folding the metal along a longitudinal fold line together near the smaller part of the tapered member.
Canadian Pat. No. 661,424 discloses a mine roof bolt expanding shell comprising two semi-cylindrical sections connected by a bridging strap, wheren each section is slotted centrally from the upper end to provide a pair of segments. Expansion in four radial directions is thus allowed, so that a more circumferential grip results. The exterior of the segments uses circumferentially extending and axially-space gripping teeth.
Canadian Pat. Nos. 1,029,221 and 1,110,091 and U.S. Pat. No. 3,250,170 disclose forming an expansion shell from a single piece of sheet metal by first cutting it to form two spade-shaped pieces connected by a narror portion which later becomes the bail.
Canadian Pat. No. 870,345 discloses a method of making expansion shells from generally planar sheet metal. An expansion shell having a plurality of toothed fingers disposed in a generally tubular configuration is formed from a uniform sheet of steel.